The present invention generally relates to bearings and more particularly, to a fluid bearing device employing lubricant, which is constituted by a thrust bearing member and a radial bearing member provided independently of each other.
One example of prior art fluid bearing devices, which is applied to a rotating head apparatus (hereinbelow, referred to as a "VTR cylinder") of a video tape recorder, is shown in FIGS. 1 to 3. In FIGS. 1 to 3, a fixed shaft 1 is press fitted into a central bore of a lower cylinder 2. A disk 3 is rotatably fitted around the fixed shaft 2. A thrust bearing plate 4 is mounted on an upper face of the disk 3, while an upper cylinder 5 is secured to a shoulder portion of the disk 3. Furthermore, a magnetic head 6 is fixed to a lower face of the upper cylinder 5. A rotary member 7 of a rotary transformer is secured to a lower portion of the disk 3 so as to transmit, through the rotating magnetic head 6, to a fixed member 8 of the rotary transformer video signals delivered from a magnetic tape. The fixed member 8 of the rotary transformer, which receives the video signals of the magnetic tape, is fixed to the lower cylinder 2 so as to confront the rotary member 7 of the rotary transformer. Thus, a rotary unit 10 is constituted by the disk 3, thrust bearing plate 4, upper cylinder 5, magnetic head 6 and rotary member 7.
Meanwhile, a C-shaped retaining ring 9 shown in FIG. 3 is fitted around an upper portion of the fixed shaft 1 so as to prevent the rotary unit 10 from being removed from the fixed shaft 1. Furthermore, an armature magnet 11 of a direct drive type motor 13 is secured to a lower portion of the disk 3. A fixed coil unit 12 is mounted on the lower cylinder 2 so as to confront the armature magnet 11 such that the direct drive type motor 13 is secured to a lower portion of the disk 3. A fixed coil unit 12 is mounted on the lower cylinder 2 so as to confront the armature magnet 11 such that the direct drive type motor 13 is constituted by the armature magnet 11 and the fixed coil unit 12. The fixed coil unit 12 is at all times attracted at a force of 600 to 900 grams by the armature magnet 11.
Moreover, radial grooves 14A and 14B are, respectively, formed at the upper portion and a central portion of the fixed shaft 1 by etching, etc. An end face 1A of the fixed shaft 1 is subjected to precision machining in flatness and perpendicularity to an axis of the fixed shaft 1. A single-row spiral groove 15 shown in FIG. 2 is formed on a central lower face of the thrust bearing plate 4 by etching, etc. so as to confront the end face 1A. Either an identical oil or an identical grease is supplied, as lubricant of the known fluid bearing device, into the radial grooves 14A and 14B and the spiral groove 15. Accordingly, when the rotary unit 10 is rotated by the motor 13, a pressure is generated by a pumping effect of the grooves 14A, 14B and 15, so that rigidity of the oil film increases and thus, the rotary unit 10 rotates relative to the fixed shaft 1 with a bearing clearance of the oil film being positively defined therebetween. It is to be noted that the known fluid bearing device is constituted by a radial bearing member 16 and a thrust bearing member 17. The radial bearing member 16 of the known fluid bearing device is constituted by the fixed shaft 1, the radial grooves 14A and 14B and the disk 3, while the thrust bearing member 17 of the known fluid bearing device is constituted by the thrust bearing plate 4, the end face 1A and the spiral groove 15.
However, in the case where an identical oil or an identical grease is supplied into the radial grooves 14A and 14B and the spiral groove 15 as described above, the prior art fluid bearing device has the following three drawbacks.
Firstly, in the case where oil is used as the lubricant, a frictional bearing torque of the known fluid bearing device is required to be lowered to a small value in order to decrease power consumption of the motor 13 acting as a driving source. A load capacity Pr of the radial bearing member 16 and a frictional bearing torque Mr of the known fluid bearing device are, respectively, given by the following equations (1) and (2): EQU Pr.varies.R.sup.2 .multidot.B.sup.2 ( 1) EQU Mr.varies.R.sup.3 .multidot.B (2)
where:
R=radius of fixed shaft 1, and PA1 B=length of known fluid bearing device.
It will be readily understood from the above equations (1) and (2) that when a value of (R/B) is decreased, the frictional bearing torque Mr can be decreased while fixing the load capacity Pr at a constant value. Meanwhile, in order to lower a production cost of the known fluid bearing device through reduction of the number of its components, it has been usually so arranged as described above that the thrust bearing member 17 is constituted by the thrust bearing plate 4 and the end face 1A. Thus, when the radius R of the fixed shaft 1 is decreased, an area of the end face 1A of the fixed shaft 1 becomes excessively small. Furthermore, since oils usually have viscosities lower than those of greases and become further less viscous at high temperatures, a sufficient bearing clearance of the oil film is not defined at the thrust bearing member 17 and thus, the thrust bearing plate 4 and the end face 1A are subjected to wear through contact therebetween.
Secondly, in the case where grease is used as the lubricant, the grease has such inferior temperature characteristics compared with a low-viscosity oil that the grease has an extremely high viscosity at low temperatures as shown in FIG. 4. Meanwhile, the radial bearing member 16 is usually subjected to not less than 80% of the frictional bearing torque Mr. Thus, in the case where the grease is used for the radial bearing member 16, the frictional bearing torque Mr increases at low temperatures, so that a torque generated by the motor 13 becomes insufficient and thus, it becomes impossible to rotate the motor 13 at a predetermined number of revolutions.
Thirdly, in view of the disadvantages of the above first and second cases, oil is used for the radial bearing member 16, while grease is used only for the thrust bearing member 17. However, in this case, when the known fluid bearing device is operated at high temperatures of about 80.degree. C. or more for a long time, a portion of base oil in the grease oozes out of the grease so as to be mixed with the oil of the radial bearing member 16 such that viscosity of the oil of the radial bearing member 16 changes or lubricating property of the oil changes extremely due to deterioration of effects of the additives, thereby impairing reliability of the known fluid bearing device.